1. Field of Invention
The invention relates to processes for using laser-scan addressed electrophotographic systems to make and assemble a number of color half-tone separation images to give a full color reproduction. The invention is particularly related to methods of color proofing. It also has application for the production of single color images on transparent substrates.
2. Background of the Art
Full color reproductions by electrophotography were disclosed by C. F. Carlson in early patents (e.g. U.S. Pat. No. 2,297,691) but no detailed mechanisms were described and the toners disclosed were dry powders. U.S. Pat. No. 2,899,335 and U.S. Pat. No. 2,907,674 pointed out that dry toners had many limitations as far as image quality is concerned especially when used for superimposed color images. They recommended the use of liquid toners for this purpose. These toners comprised a carrier liquid which were of high resistivity e.g. 10.sup.9 ohm-cm or more, having colorant particles dipersed in the liquid, and preferably including an additive intended to impart the charge carried by the colorant particles. U.S. Pat. No. 3,337,340 disclosed that one toner deposited first may be sufficiently conductive to interfere with a succeeding charging step; it claimed the use of insulative resins (resistivity greater than 10.sup.10 ohm-cm) of low dielectric constant (less than 3.5) covering each colorant particle. U.S. Pat. No. 3,135,695 disclosed toner particles stably dispersed in an insulating aliphatic liquid, the toner particles comprising a charged colorant core encapsulated by an aromatic soluble resin treated with a small quantity of an aryl-alkyl material. The use of metal soaps as charge control and stabilizing additives to liquid toners is disclosed in many earlier patents (e.g. U.S. Pat. No. 3,900,412; U.S. Pat. No. 3,417,019; U.S. Pat. No. 3,779,924; U.S. Pat. No. 3,788,995). On the other hand, concern is expressed and cures offered for the inefficient action experienced when charge control or other charged additives migrate from the toner particles into the carrier liquid (U.S. Pat. No. 3,900,413; U.S. Pat. No. 3,954,640; U.S. Pat. No. 3,977,983; U.S. Pat. No. 4,081,391; U.S. Pat. No. 4,264,699). In U.S. Pat. No. 3,890,240 it is disclosed that typical liquid toners known in the art have conductivities in the range 1.times.10.sup.-11 to 10.times.10.sup.-11 mho/cm. A British patent (GB 2,023,860) discloses centrifuging the toner particles out of a liquid toner and redispersing them in fresh liquid as a way of reducing conductivity in the liquid itself. After repeating the process several times the conductivity of the liquid toner was reduced by a factor of about 23 and is disclosed as a sensitive developer for low contrast charge images. In several patents the idea is advanced that the level of free charge within the liquid toner as a function of the mass of toner particles is important to the efficiency of the developing process. In U.S. Pat. No. 4,547,449 this measure was used to evaluate the unwanted charge buildup on replenishment of the toner during use, and in U.S. Pat. No. 4,606,989 it was used as a measure of deterioration of the toner on aging. In U.S. Pat. No. 4,525,446 the aging of the toner was measured by the charge present which was generally related to the zeta potential of the individual particles. A related patent, U.S. Pat. No. 4,564,574, discloses chelating charge director salts onto the polymer and discloses measured values of zeta potential on toner particles. Values of 33 mV and 26.2 mV with particle diameters of 250 nm and 400 nm are given. The import of this patent is improved stability of the liquid toner. A literature reference by Muller et al in 1980 (Research into the Electrokinetic Properties of Electrographic Liquid Developers, V. M. Muller et al, IEEE Transactions on Industry Applications, vol IA-16, pages 771-776 (1980)) treats the liquid toner system theoretically but also gives experimental results on certain toners. Using very small toner particles (all less than about 0.1 micron) they present zeta potentials in the range 15 mV to 99 mV with related conductivity ratios. These latter ratios appear, however, to relate the conductivity of the toner immediately after the current is initiated to the value after prolonged passage of the current. The former is believed to contain both toner particle and soluble ionic species conductivities; the latter is believed to be the basic conductivity of the carrier liquid after most of the added charged carriers have been deposited by the current flow. Finally in U.S. Pat. No. 4,155,862 the charge per unit mass of the toner was related to difficulties experienced in the art in superposing several layers of different colored toners. This latter problem was approached in a different way in U.S. Pat. No. 4,275,136 where adhesion of one toner layer to another was enhanced by aluminum or zinc hydroxide additives on the surface of the toner particles.
Diameters of toner particles in liquid toners vary from a range of 2.5 to 25.0 microns in U.S. Pat. No. 3,900,412 to values in the sub-micron range in U.S. Pat. Nos. 4,032,463, 4,081,391, and 4,525,446, and are even smaller in the Muller paper (supra). It is stated in U.S. Pat. No. 4,032,463 that the prior art makes it clear that sizes in the range 0.1 to 0.3 microns are not preferred because they give low image densities.
Liquid toners which provide developed images which rapidly self-fix to a smooth surface at room temperature after removal of the carrier liquid are disclosed in U.S. Pat. No. 4,480,022 and U.S. Pat. No. 4,507,377. These toner images are said to have higher adhesion to the substrate and to be less liable to crack. No disclosure is made of their use in multicolor image assemblies.
Explicit references to liquid developer compositions designed for use with half-tone images are not common in the art. Thus U.S. Pat. Nos. 3,594,161, 4,182,266, 4,358,195, 4,510,223, 4,547,061, and 4,556,309 all disclose electrophotographic systems designed for half-tone images without giving details of the composition of the toners used. In U.S. Pat. No. 4,640,605, no details of toner constitution are given, but development conditions like bias field are specified without relating them to the toner parameters. In U.S. Pat. No. 4,657,831, again no toner details are given, but optical modification of developed multicolor half-tone dots is disclosed to simulate in a proof the dot gain found on printing. EPA 85301933.9 describes the influence on tone reproduction in half-tone images of the statistical distribution of charges on the toner particles but gives no details of other constitutional parameters. Only U.S. Pat. No. 4,600,669 provides details of liquid toners for use in half-tone image proofing; these toners contain toner particles comprising colorant, polyester binder, a wax and a wax dispersant, the particles being suspended in an insulating carrier liquid.
The art therefore discloses a consciousness of the importance of the physical parameters of the liquid toner--conductivities, zeta potentials of toner particles, charge per particle or per unit mass of particles, and the localization of the charge on the particles. Most of the references above are concerned with the efficiency of liquid toners in the context of monochromatic image development. Of those giving any appreciable details of the toners used, only U.S. Pat. Nos. 4,155,862, 4,275,136, and 4,600,669 are explicitly concerned with multicolor toned images, and only the first of these relates the quality of the multicolor toned assembly to parameters such as the charge per gram of the toner particles.
Other features of electrophotographic imaging are known and are taught in references such as
U.S. Pat. No. 3,248,216 describes halftoning an image to reduce the electrophotographic contrast.
U.S. Pat. No. 3,362,907 describes a liquid developer with sharp cut off response that uses a sensitizing agent to adjust contrast.
U.S. Pat. No. 3,560,203 and U.S. Pat. No. 3,784,397 discuss development and edge enhancement.
U.S. Pat. No. 3,635,195 describes producing halftone prints with a developer that contains an array of projections. High fields are used (close spacing).
U.S. Pat. No. 3,707,139 discusses the flow of toners through a gap and the spacing to affect development.
U.S. Pat. No. 3,766,072 describes a method to reduce edge effect with a two pigment developer that varies in conductivity.
U.S. Pat. No. 3,799,791 describes a field controlled development where the photoreceptor is held away from the developer by the liquid (thus narrow gap)
U.S. Pat. No. 3,817,748 describes contrast control with polar liquid imaging.
U.S. Pat. No. 4,023,900 describes adjusting the contrast by process conditions. However this is specifically applied to patterned application of polar liquids.
U.S. Pat. No. 4,623,241 discusses some interactive effects to optimize development density.
U.S. Pat. No. 4,648,704 describes development conditions where lower concentration toners are described as capable of developing small image detail with greater density and sharper edges. Research disclosure 167823 discusses dry toner conductivity to adjust edge enhancement and copy contrast.